The present invention relates to surface-crosslinked superabsorbent polymer particles, and to methods of producing the surface-crosslinked super-absorbent particles. The present invention also relates to the use of the surface-crosslinked particles in articles, such as diapers, catamenial devices, and wound dressings. More particularly, the present invention relates to surface treating superabsorbent polymer (SAP) particles, such as a neutralized, crosslinked, homopolymer or copolymer of acrylic acid, with an oxazolinium ion to substantially improve the water absorption and water retention properties of the SAP particles.
Water-absorbing resins are widely used in sanitary goods, hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and controlled release agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles.
Such water-absorbing resins are termed xe2x80x9csuperabsorbent polymers,xe2x80x9d or SAPS, and typically are lightly crosslinked hydrophilic polymers. SAPs are generally discussed in Goldman et al. U.S. Pat. Nos. 5,669,894 and 5,559,335, the disclosures of which are incorporated herein by reference. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirements for an SAP used in a hygienic article, such as a diaper.
As used here and hereafter, the term xe2x80x9cSAP particlesxe2x80x9d refers to superabsorbent polymer particles in the dry state, i.e., particles containing from no water up to an amount of water less than the weight of the particles. The term xe2x80x9cparticlesxe2x80x9d refers to granules, fibers, flakes, spheres, powders, platelets, and other shapes and forms known to persons skilled in the art of superabsorbent polymers. The terms xe2x80x9cSAP gelxe2x80x9d and xe2x80x9cSAP hydrogelxe2x80x9d refer to a superabsorbent polymer in the hydrated state, i.e., particles that have absorbed at least their weight in water, and typically several times their weight in water. The terms xe2x80x9csurface-treated SAP particlexe2x80x9d and xe2x80x9csurface-crosslinked SAP particlexe2x80x9d refer to an SAP particle having its molecular chains present in the vicinity of the particle surface crosslinked by a compound applied to the surface of the particle. The term xe2x80x9csurface crosslinkingxe2x80x9d means that the level of functional crosslinks in the SAP particle in the vicinity of the surface of the particle is generally higher than the level of functional crosslinks in the SAP particle in the interior of the particle.
SAP particles can differ in ease and cost of manufacture, chemical identity, physical properties, rate of water absorption, and degree of water absorption and retention, thus making the ideal water-absorbent resin a difficult composition to design. For example, the hydrolysis products of starch-acrylonitrile graft polymers have a comparatively high ability to absorb water, but require a cumbersome process for production and have the disadvantages of low heat resistance and decay or decomposition due to the presence of starch. Conversely, other water-absorbent polymers are easily and cheaply manufactured and are not subject to decomposition, but do not absorb liquids as well as the starch-acrylonitrile graft polymers.
Therefore, it would be extremely advantageous to provide a method of increasing the water absorption properties of a stable, easy to manufacture SAP particles to match the superior water absorption properties of a difficult to manufacture particle. Likewise, it would be advantageous to further increase the liquid absorption properties of already superior SAP particles.
In addition, conventional SAP particles all have a serious defect in that their rates of liquid absorption are lower than fluff pulp and paper. For example, when urine is excreted on a disposable diaper containing conventional SAP particles, the urine can remain in contact with the skin for a relatively long time and make the wearer uncomfortable. This is attributed to the low rate at which the diaper can absorb urine.
Attempts have been made to increase the liquid absorption rate by increasing the surface area of the SAP particle, i.e., by decreasing its particle size. However, when the particle size of the SAP particle is decreased, it generally forms a xe2x80x9cfish eyexe2x80x9d upon contact with urine, which retards the speed of liquid absorption. When the SAP particles are in the form of granules, each granule constitutes a xe2x80x9cfish eyexe2x80x9d and the speed of liquid absorption decreases. SAP particles in flake form exhibit a moderate increase in the speed of liquid absorption, but SAP flakes are bulky and are difficult to transport and store.
Initially, the swelling capacity of an SAP particle on contact with liquids, also referred to as free swelling capacity, was the main factor in the design and development of SAP particles. Later, however, it was found that not only is the amount of absorbed liquid important, but the stability of the swollen gel, or gel strength, also is important. The free swelling capacity, on one hand, and the gel strength, on the other hand, represent contrary properties. Accordingly, SAP particles having a particularly high absorbency typically exhibit a poor gel strength, such that the gel deforms under pressure (e.g., the load of a body), and prevents further liquid distribution and absorption.
A balanced relation between absorptivity (gel volume) and gel strength is desired to provide proper liquid absorption, liquid transport, and dryness of the diaper and the skin when using SAP particles in a diaper. In this regard, not only is the ability of the SAP particle to retain a liquid under subsequent pressure an important property, but absorption of a liquid against a simultaneously acting pressure, i.e., during liquid absorption also is important. This is the case in practice when a child or adult sits or lies on a sanitary article, or when shear forces are acting on the sanitary article, e.g., leg movements. This absorption property is referred to as absorption under load.
Currently, there is a trend to reduce the size and thickness of sanitary articles for esthetic and environmental reasons (e.g., reduction of waste in landfills). This is accomplished by reducing the large volume of fluff pulp and paper in diapers, and increasing the amount of SAP particles. The SAP particles, therefore, have to perform additional functions with respect to liquid absorption and transport which previously were performed by the fluff pulp and paper, and which could not be accomplished satisfactorily with conventional SAP particles.
Investigators have researched various methods of improving the amount of liquid absorbed and retained by SAP particles, especially under load, and the rate at which the liquid is absorbed. One preferred method of improving the absorption and retention properties of SAP particles is to surface treat the SAP particles.
The surface treatment of SAP particles is well known. For example, U.S. Pat. No. 4,043,952 discloses the use of polyvalent metal compounds as surface treating compounds. U.S. Pat. No. 4,051,086 discloses the use of glyoxal as a surface treatment to improve the absorption rate of SAP particles. The surface treatment of SAP particles with crosslinking agents having two or more functional groups capable of reacting with pendant carboxylate or other groups contained on the polymer comprising the SAP particle is disclosed in various patents. The surface treatment improves absorbency and gel rigidity to improve liquid flowability and prevent SAP particle agglomeration, as well as improving gel strength.
As disclosed in the art, the SAP particles are either mixed with the surface-crosslinking agent, optionally using small amounts of water and/or an organic solvent, or an SAP hydrogel containing 10% to 40%, by weight, water is dispersed in a hydrophilic or hydrophobic solvent and mixed with the surface-crosslinking agent.
Prior surface crosslinking agents include diglycidyl ethers, halo epoxy compounds, polyols, polyamines, polyisocyanates, polyfunctional aziridine compounds, and di- or tri-alkylhalides. Regardless of the identity of the surface crosslinking agent, the agent used for the surface treatment has at least two reactive functional groups, and the SAP particles are heated after the surface crosslinking agent is applied to the surface of the SAP particles.
Surface-crosslinked SAP particles, in general, exhibit higher liquid absorption and retention values than SAP particles having a comparable level of internal crosslinks, but lacking surface crosslinking. Internal crosslinks arise from polymerization of the monomers comprising the SAP particles, and are present in the polymer backbone. It has been theorized that surface crosslinking increases the resistance of SAP particles to deformation, thus reducing the degree of contact between surfaces of neighboring SAP particles when the resulting hydrogel is deformed under an external pressure. The degree to which absorption and retention values are enhanced by surface crosslinking is related to the relative amount and distribution of internal and surface crosslinks, and to the particular surface crosslinking agent and method of surface crosslinking.
As understood in the art, surface-crosslinked SAP particles have a higher level of crosslinking in the vicinity of the surface than in the interior. As used herein, xe2x80x9csurfacexe2x80x9d describes the outer-facing boundaries of the particle. For porous SAP particles, exposed internal surface also are included in the definition of surface.
Prior methods of performing surface crosslinking of SAP particles are disclosed, for example, in Obayashi U.S. Pat. No. 4,541,871, WO 92/16565, WO 93/05080, Alexander U.S. Pat. No. 4,824,901, Johnson U.S. Pat. No. 4,789,861, Makita U.S. Pat. No. 4,587,308, Tsubakimoto U.S. Pat. No. 4,734,478, Kimura et al. U.S. Pat. No. 5,164,459, DE 4,020,780, and EPO 509,708. Surface crosslinking of SAPs is generally discussed in F. L. Buchholz et al., ed., xe2x80x9cModern Superabsorbent Polymer Technology,xe2x80x9d Wiley-VCH, New York, NY, pages 97-108 (1998).
A problem encountered in several prior compounds and methods used to surface crosslink SAP particles is the relatively high temperature required to form the surface crosslinks between the SAP and the surface crosslinking agent. Typically, temperatures in excess of 180xc2x0 C. are required to form the surface crosslinks. At such temperatures, the SAP particle has a tendency to degrade in color from white or off-white to tan or brown. Such color degradation provides an SAP particle that is esthetically unacceptable to consumers. In addition, a high surface crosslinking temperature can increase the residual monomer content of the SAP particle, which can lead to adverse environmental and health effects, or can lead to rejection of the SAP particles for failing to meet specifications.
The present invention is directed to a more reactive class of crosslinking agents, that does not require the high temperatures or prolonged heating associated with prior surface crosslinking agents. The present invention also is directed to surface-treated SAP particles that overcome the disadvantages associated with the use of prior surface crosslinking agents and with prior surface crosslinked SAP particles.
The present invention is directed to surface-treated SAP particles and to a method of surface treating SAP particles with a sufficient amount of an oxazolinium ion to substantially improve the water-absorption and water retention properties of the SAP particles. In particular, the present invention is directed to treating SAP particles with oxazolinium ions, which can be formed in situ, by applying an oxazolinium ion precursor, such as a hydroxyalkylamide (HAA), to the surface of the SAP particles, then heating the precursor-treated SAP particles at about 90xc2x0 C. to about 170xc2x0 C. for about 60 to about 180 minutes to form the oxazolinium ion, which then surface crosslinks the SAP particles.
If the oxazolinium ion has sufficient stability to exist for a prolonged time at room temperature, then SAP particles can be treated with the oxazolinium ion, and the resulting treated particles can be heated to surface crosslink the SAP particles. In this embodiment, the stable oxazolinium ion is sufficiently reactive to surface crosslink the SAP particles at a temperature of about 90xc2x0 C. to about 170xc2x0 C.
In accordance with the present invention, SAP particles possess improved water absorption and water retention properties as a result of surface treatment with an oxazolinium ion. Treatment with an oxazolinium ion is especially effective when performed on polyacrylate salts, hydrolyzed polyacrylamides, or other polymers having a plurality of pendent neutralized carboxyl groups.
Therefore, the present invention is directed to surface-treated SAP particles having about 0.001 to about 10 parts by weight of an oxazolinium ion, or oxazolinium ion precursor, per 100 parts by weight of SAP particles, applied to the surfaces of the SAP particles to crosslink molecular chains existing at least in the vicinity of the surface of the SAP particles.
One aspect of the present invention is to provide such surface-treated SAP particles, and to a method of manufacturing the surface-treated SAP particles comprising applying a sufficient amount of an HAA to surfaces of the SAP particles and heating the surface-treated SAP particles at a sufficient temperature and for a sufficient time for the HAA to form an oxazolinium ion, which reacts with pendent groups on a polymer comprising the SAP particle to form surface crosslinks on the SAP particle.
Yet another aspect of the present invention is to provide a method of surface crosslinking SAP particles comprising applying a sufficient amount of an oxazolinium ion to surfaces of the SAP particles and heating the surface treated SAP particles at about 90xc2x0 C. to about 170xc2x0 C. for about 60 to about 180 minutes for the oxazolinium ion to surface crosslink the SAP particles.
Another aspect of the present invention is to provide a method of surface treating SAP particles comprising heating a sufficient amount of an HAA in the presence of the SAP particles at a sufficient temperature (e.g., about 100xc2x0 C. to about 160xc2x0 C. and for a sufficient time (e.g., about 90 to about 150 minutes) for the HAA to cyclize and thereby form an oxazolinium ion, which then surface crosslinks the SAP particles.
Yet another aspect of the present invention is to provide surface-treated SAP particles exhibiting a high retention capacity, high gel strength, and high absorbency under load. This aspect is achieved by exposing a particle-shaped SAP to about 0.001% to about 10% by weight of an oxazolinium ion, either formed in situ by heating an HAA in the presence of SAP particles, or by applying an oxazolinium ion to the particles, and heating to about 90xc2x0 C. to about 170xc2x0 C.
Another aspect of the present invention is to provide an SAP particle having surface crosslinks provided by an oxazolinium ion, which is generated by heating a hydroxyalkylamide (HAA) having the structure: 
wherein A is a bond, hydrogen, or a monovalent or polyvalent organic radical selected from the group consisting of a saturated or unsaturated alkyl radical containing 1 to 60 carbon atoms, aryl, tri-C1-4alkyleneamino, and an unsaturated radical containing one or more ethylenic groups [ greater than C=C less than ]; R1, selected independently, are hydrogen, straight or branched chain C1-5alkyl, or straight or branched chain C1-5hydroxyalkyl; R2, selected independently, are radicals selected from the group consisting of hydrogen and straight or branched chain C1-5alkyl, or the R2 radicals can be joined to form, together with the carbon atoms, a cycloalkyl ring; p and pxe2x80x2, independently, are integers 1 to 4; n is an integer having a value of 1 or 2, and nxe2x80x2 is an integer having a value 0 to 2, or when nxe2x80x2 is 0, a polymer or copolymer (i.e., n has a value greater than 1, preferably 2 to 10) formed from the hydroxyalkylamide when A is an unsaturated radical.
Another aspect of the present invention is to provide an SAP particles having surface crosslinks provided by an oxazolinium having the structure: 
wherein B is 
xe2x80x83and A, R1, R2, p, p1, and n are as defined above with respect to an HAA. Such oxazolinium ions are stable at room temperature, and have a counterion such as tosylate, mesylate, or fluoride. The oxazolinium ions are sufficiently reactive to form surface crosslinks on an SAP particle at about 90xc2x0 C. to about 170xc2x0 C.
Still another aspect of the present invention is to provide SAP particles surface crosslinked with a oxazolinium ion in an amount sufficient to substantially improve the water absorbency and water retention properties of the SAP particles, such as retention capacity, absorption rate, and gel strength, and to maintain a xe2x80x9cdry feelxe2x80x9d for the SAP particles after significant liquid absorption.
These and other aspects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments.
In accordance with the present invention, SAP particles are surface treated with an oxazolinium ion to substantially increase the rate of liquid absorption, amount of liquid absorption, and overall retention of liquids by the SAP particles. Surface-treatment of the SAP particles at any time after polymerization and sufficient drying to form solid SAP particles improves liquid absorption properties. For economics and ease of manufacture, the surface treatment is performed most advantageously immediately after the SAP particles are synthesized, dried to an appropriate water content, and sized, such as by grinding.
As will become apparent from the following detailed description of the preferred embodiments, surface-crosslinking of SAP particles with an oxazolinium ion substantially improves the water absorption properties of the SAP particles, which can be either acidic or basic in nature. Of particular utility are SAP particles containing a plurality of pendant, neutralized carboxyl groups along the polymer chain.
As stated above, the surface treatment of SAP particles is well known. However, many surface crosslinking agents exhibit disadvantages. Some surface crosslinking agents have toxic properties and, therefore, cannot be used in the sensitive field of hygiene because they pose a threat to health or the environment. For example, in addition to the risk of skin irritation, epoxy, glycidyl, isocyanate, and organic halogen compounds, have a sensitizing effect, and frequently have a carcinogenic and mutagenic potential. Polyamines are avoided as surface crosslinking agents because of possible nitrosamine formation. In any case, when used in diapers and other sanitary articles, residual amounts of toxicologically critical crosslinking agents must be removed from the SAP particles, which involves additional process steps and increases the cost of the SAP particles.
In addition, a majority of the commonly used surface crosslinking agents required heating at temperatures in excess of 180xc2x0 C. in order to react with the SAP particles and form surface crosslinks. Heating at such a high temperature can increase the residual monomer content of the surface crosslinked SAP particles. An increased residual monomer content poses both toxicological and environmental concerns, and is unacceptable commercially.
The high temperature required to form surface crosslinks also causes the SAP particles to degrade in color from white or off-white to tan or brown. The tan to brown color of the SAP particles is esthetically unacceptable to consumers, who equate the tan to brown color of the SAP particles to an inferior product. The combination of color degradation and increased residual monomer can lead to SAP particles that do not meet production specifications and, therefore, are refused by the purchaser and/or consumer. The present invention overcomes these disadvantages associated with prior surface crosslinking agents by utilizing an oxazolinium ion as the surface crosslinking agent for the SAP particles.
The identity of the SAP particles utilized in the present invention is not limited. The SAP particles are prepared by methods well known in the art, for example, solution or emulsion polymerization. The SAP particles, therefore, can comprise an acidic water-absorbing resin, a basic water-absorbing resin, a blend of an acidic and basic water-absorbing resin, or a multicomponent SAP particle as disclosed in WO 99/25393, the disclosure of which is incorporated herein by reference.
The SAP particles are prepared, for example, by:
(1) copolymerizing an acrylate salt and a crosslinking monomer in aqueous solution, and drying the resulting gel-like hydrous polymer by heating;
(2) dispersing an aqueous solution of acrylic acid and/or an alkali metal acrylate, a water-soluble radical polymerization initiator, and a crosslinkable monomer in an alicyclic and/or an aliphatic hydrocarbon solvent in the presence of a surface-active agent, and subjecting the mixture to suspension polymerization;
(3) saponifying copolymers of vinyl esters and ethylenically unsaturated carboxylic acids or their derivatives;
(4) polymerizing starch and/or cellulose, a monomer having a carboxyl group or capable of forming a carboxyl group upon hydrolysis, and a crosslinking monomer in an aqueous medium, and, as required, hydrolyzing the resulting polymer; or
(5) reacting an alkaline substance with a maleic anhydride-type copolymer containing maleic anhydride and at least one monomer selected from xcex1-olefins and vinyl compounds, and, as required, reacting the reaction product with a polyepoxy compound. Other methods and monomers that provide SAP particles also are known in the art.
Generally, acidic water-absorbing resins have carboxylate, sulfonate, sulfate, and/or phosphate groups incorporated along the polymer chain. Polymers containing these acid moieties are synthesized either from monomers previously substituted with one or more of these acidic functional groups or by incorporating the acidic functional group into the polymer after synthesis. To incorporate carboxyl groups into a polymer, any of a number of ethylenically unsaturated carboxylic acids can be homopolymerized or copolymerized. Carboxyl groups also can be incorporated into the polymer chain indirectly by hydrolyzing a homopolymer or copolymer of monomers such as acrylamide, acrylonitrile, methacrylamide, and alkyl acrylates or methacrylates.
An acidic water-absorbing resin present in an SAP particle can be either a strong or a weak acidic water-absorbing resin. The acidic water-absorbing resin can be a single resin, or a mixture of resins. The acidic resin can be a homopolymer or a copolymer.
The acidic water-absorbing resin typically is a neutralized, lightly crosslinked acrylic-type resin, such as neutralized, lightly crosslinked polyacrylic acid. The lightly crosslinked acidic resin typically is prepared by polymerizing an acidic monomer containing an acyl moiety, e.g., acrylic acid, or a moiety capable of providing an acid group, i.e., acrylonitrile, in the presence of a free radical crosslinker, i.e., a polyfunctional organic compound. The acidic resin can contain other copolymerizable units, i.e., other monoethylenically unsaturated comonomers, well known in the art, as long as the polymer is substantially, i.e., at least 10%, and preferably at least 25%, acidic monomer units. To achieve the full advantage of the present invention, the acidic resin contains at least 50%, and more preferably, at least 75%, and up to 100%, acidic monomer units. The acidic resin is neutralized at least 50 mole %, and preferably at least 70 mole %, with a base prior to surface crosslinking.
Ethylenically unsaturated carboxylic acid and carboxylic acid anhydride monomers, and salts, useful in the acidic water-absorbing resin include acrylic acid, methacrylic acid, ethacrylic acid, xcex1-chloroacrylic acid, xcex1-cyanoacrylic acid, xcex2-methylacrylic acid (crotonic acid), xcex1-phenylacrylic acid, xcex2-acryloxypropionic acid, sorbic acid, xcex1-chlorosorbic acid, angelic acid, cinnamic acid, p-chlorocinnamic acid, xcex2-stearylacrylic acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, maleic acid, fumaric acid, tricarboxyethylene, 2-methyl-2-butene dicarboxylic acid, maleamic acid, N-phenyl maleamide, maleamide, maleic anhydride, fumaric anhydride, itaconic anhydride, citraconic anhydride, mesaconic anhydride, methyl itaconic anhydride, ethyl maleic anhydride, diethylmaleate, methylmaleate, and maleic anhydride.
Sulfonate-containing acidic resins can be prepared from monomers containing functional groups hydrolyzable to the sulfonic acid form, for example, alkenyl sulfonic acid compounds and sulfoalkylacrylate compounds. Ethylenically unsaturated sulfonic acid monomers include aliphatic or aromatic vinyl sulfonic acids, such as vinylsulfonic acid, allyl sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, acrylic and methacrylic sulfonic acids, such as sulfoethyl acrylate, sulfoethyl methacrylate, sulfopropyl acrylate, 2-vinyl-4-ethylbenzene, 2-allylbenzene sulfonic acid, 1-phenylethylene sulfonic acid, sulfopropyl methacrylate, 2-hydroxy-3-methacryloxypropyl sulfonic acid, and 2-acrylamide-2-methylpropane sulfonic acid.
Sulfate-containing acidic resins are prepared by reacting homopolymers or copolymers containing hydroxyl groups or residual ethylenic unsaturation with sulfuric acid or sulfur trioxide. Examples of such treated polymers include sulfated polyvinylalcohol, sulfated hydroxyethyl acrylate, and sulfated hydroxypropyl methacrylate. Phosphate-containing acidic resins are prepared by homopolymerizing or copolymerizing ethylenically unsaturated monomers containing a phosphoric acid moiety, such as methacryloxy ethyl phosphate.
Copolymerizable monomers for introduction into the acidic resin, or into the basic resin, include, but are not limited to, ethylene, propylene, isobutylene, C1 to C4 alkyl acrylates and methacrylates, vinyl acetate, methyl vinyl ether, and styrenic compounds having the formula: 
wherein R represents hydrogen or a C1-6 alkyl group, and wherein the phenyl ring optionally is substituted with one to four C1-4 alkyl or hydroxy groups.
Suitable C1 to C4 alkyl acrylates include, but are not limited to, methyl acrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butyl acrylate, and the like, and mixtures thereof. Suitable C1 to C4 alkyl methacrylates include, but are not limited to, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-propylmethylmethacrylate, n-butyl methacrylate, and the like, and mixtures thereof or with C1-4 alkyl acrylates. Suitable styrenic compounds include, but are not limited to, styrene, xcex1-methylstyrene, p-methylstyrene, t-butyl styrene, and the like, and mixtures thereof or with C1-4 alkyl acrylates and/or methacrylates.
As set forth above, polymerization of acidic monomers, and optional copolymerizable monomers, most commonly is performed by free radical processes in the presence of a polyfunctional organic compound. The acidic resins are crosslinked to a sufficient extent such that the polymer is water insoluble. Crosslinking renders the acidic resins substantially water insoluble, and, in part, serves to determine the absorption capacity of the resins. For use in absorption applications, an acidic resin is lightly crosslinked, i.e., has a crosslinking density of less than about 20%, preferably less than about 10%, and most preferably about 0.01% to about 7%.
A crosslinking agent most preferably is used in an amount of less than about 7 wt %, and typically about 0.1 wt % to about 5 wt %, based on the total weight of monomers. Examples of crosslinking polyvinyl monomers include, but are not limited to, polyacrylic (or polymethacrylic) acid esters represented by the following formula (I); and bisacrylamides, represented by the following formula (II). 
wherein X is ethylene, propylene, trimethylene, cyclohexyl, hexamethylene, 2-hydroxypropylene, xe2x80x94(CH2CH2O)pCH2CH2xe2x80x94, or 
p and r are each an integer 5 to 40, and k is 1 or 2; 
wherein 1 is 2 or 3.
The compounds of formula (I) are prepared by reacting polyols, such as ethylene glycol, propylene glycol, trimethylolpropane, 1,6-hexanediol, glycerin, pentaerythritol, polyethylene glycol, or polypropylene glycol, with acrylic acid or methacrylic acid. The compounds of formula (II) are obtained by reacting polyalkylene polyamines, such as diethylenetriamine and triethylenetetramine, with acrylic acid.
Specific crosslinking monomers include, but are not limited to, 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,3-butylene glycol diacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol dimethacrylate, ethoxylated bisphenol A diacrylate, ethoxylated bisphenol A dimethacrylate, ethylene glycol dimethacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol dimethacrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, tripropylene glycol diacrylate, tetraethylene glycol diacrylate, tetraethylene glycol dimethacrylate, dipentaerythritol pentaacrylate, pentaerythritol tetraacrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, tris(2-hydroxyethyl)isocyanurate triacrylate, tris(2-hydroxyethy)isocyanurate trimethacrylate, divinyl esters of a polycarboxylic acid, diallyl esters or a polycarboxylic acid, triallyl terephthalate, diallyl maleate, diallyl fumarate, hexamethylenebismaleimide, trivinyl trimellitate, divinyl adipate, diallyl succinate, a divinyl ether of ethylene glycol, cyclopentadiene diacrylate, tetraallyl ammonium halides, or mixtures thereof. Compounds such as divinylbenzene and divinyl ether also can be used to crosslink the poly(dialkylaminoalkyl acrylamides). Especially preferred crosslinking agents are N,Nxe2x80x2-methylenebisacrylamide, N,Nxe2x80x2-methylenebismethacrylamide, ethylene glycol dimethacrylate, and trimethylolpropane triacrylate.
The acidic resin, either strongly acidic or weakly acidic, can be any resin that acts as an SAP in its neutralized form. Examples of acidic resins include, but are not limited to, polyacrylic acid, hydrolyzed starch-acrylonitrile graft copolymers, starch-acrylic acid graft copolymers, saponified vinyl acetate-acrylic ester copolymers, hydrolyzed acrylonitrile copolymers, hydrolyzed acrylamide copolymers, ethylene-maleic anhydride copolymers, isobutylene-maleic anhydride copolymers, poly(vinylsulfonic acid), poly(vinylphosphonic acid), poly(vinylphosphoric acid), poly(vinylsulfuric acid), sulfonated polystyrene, poly(aspartic acid), poly(lactic acid), and mixtures thereof. The preferred acidic resins are the polyacrylic acids.
The final acidic SAP particle contains from about 50 to 100 percent neutralized pendant carboxylate salt units. Accordingly, it may be necessary to neutralize carboxylic acid groups. Neutralization of carboxylic acid groups is accomplished using a strong organic or inorganic base, such as sodium hydroxide, potassium hydroxide, ammonia, ammonium hydroxide, or an organic amine.
The sequence and the number of reactions (e.g., polymerization, hydrolysis, and neutralization) performed to obtain the desired acid functionality attached to acidic resin backbone are not critical. Any number and sequence resulting in a final SAP particle which possesses 0 to about 90 percent copolymerizable monomer units and about 10 to about 100 percent monomer units having pendant acid groups, and neutralized at least 50 mole %, is suitable.
Analogous to the acidic resin, a basic water-absorbing resin present in the SAP particles can be a strong or weak basic water-absorbing resins. The basic water-absorbing resin can be a single resin or a mixture of resins. The basic resin can be a homopolymer or a copolymer. The identity of the basic resin is not limited as long as the basic resin is capable of reacting with an oxazolinium ion. The strong basic resins typically are present in the hydroxide (OH) or bicarbonate (HCO3) form.
The basic water-absorbing resin can be a lightly crosslinked acrylic-type resin, such as a poly(dialkylaminoalkyl (meth)acrylamide). The basic resin also can be a polymer such as a lightly crosslinked polyethylenimine, a poly(vinylamine), a poly(allylamine), a poly(allylguanidine), a poly(dimethyldiallylammonium hydroxide), a quaternized polystyrene derivative, such as 
a guanidine-modified polystyrene, such as 
a quaternized poly((meth)acrylamide) or ester analog, such as 
or 
wherein Me is methyl, R4 is hydrogen or methyl, n is a number 1 to 8, and q is a number from 10 to about 100,000, or a poly(vinylguanidine), i.e., poly(VG), a strong basic water-absorbing resin having the general structural formula (III) 
wherein q is a number from 10 to about 100,000, and R5 and R6, independently, are selected from the group consisting of hydrogen, C1-C4 alkyl, C3-C6 cycloalkyl, benzyl, phenyl, alkyl-substituted phenyl, naphthyl, and similar aliphatic and aromatic groups. The lightly crosslinked basic water-absorbing resin can contain other copolymerizable units and is crosslinked using a polyfunctional organic compound, as set forth above with respect to the acidic water-absorbing resin.
A basic water-absorbing resin used in the present SAP particles typically contains an amino or a guanidine group. Accordingly, a water-soluble basic resin can be crosslinked in solution by suspending or dissolving an uncrosslinked basic resin in an aqueous or alcoholic medium, then adding a di- or polyfunctional compound capable of crosslinking the basic resin by reaction with the amino groups of the basic resin. Such crosslinking agents include, for example, multifunctional aldehydes (e.g., glutaraldehyde), multifunctional acrylates (e.g., butanediol diacrylate, TMPTA), halohydrins (e.g., epichlorohydrin), dihalides (e.g., dibromopropane), disulfonate esters (e.g., ZS(O2)Oxe2x80x94(CH2)nxe2x80x94OF(O)2Z, wherein n is 1 to 10, and Z is methyl or tosyl), multifunctional epoxies (e.g., ethylene glycol diglycidyl ether), multifunctional esters (e.g., dimethyl adipate), multifunctional acid halides (e.g., oxalyl chloride), multifunctional carboxylic acids (e.g., succinic acid), carboxylic acid anhydrides (e.g., succinic anhydride), organic titanates (e.g., TYZOR AA from DuPont), melamine resins (e.g., CYMEL 301, CYMEL 303, CYMEL 370, and CYMEL 373 from Cytec Industries, Wayne, N.J.), hydroxymethyl ureas (e.g., N,Nxe2x80x2-dihydroxymethyl-4,5-dihydroxyethyleneurea), and multifunctional isocyanates (e.g., toluene diisocyanate or methylene diisocyanate). Crosslinking agents for basic resins also are disclosed in Pinschmidt, Jr. et al. U.S. Pat. No. 5,085,787, incorporated herein by reference, and in EP 450 923.
Conventionally, the crosslinking agent is water or alcohol soluble, and possesses sufficient reactivity with the basic resin such that crosslinking occurs in a controlled fashion, preferably at a temperature of about 25xc2x0 C. to about 150xc2x0 C. Preferred crosslinking agents are ethylene glycol diglycidyl ether (EGDGE), a water-soluble diglycidyl ether, and a dibromoalkane, an alcohol-soluble compound.
The basic resin, either strongly or weakly basic, therefore, can be any resin that acts as an SAP in its charged form. Examples of basic resins include a poly(vinylamine), a polyethylenimine, a poly(vinylguanidine), a poly(allylamine), a poly(allylguanidine), or a poly(dialkylaminoalkyl(meth)acrylamide) prepared by polymerizing and lightly crosslinking a monomer having the structure 
or its ester analog 
wherein R7 and R8, independently, are selected from the group consisting of hydrogen and methyl, Y is a divalent straight chain or branched organic radical having 1 to 8 carbon atoms, R9 is hydrogen, and R10 is hydrogen or an alkyl radical having 1 to 4 carbon atoms. Preferred basic resins include a poly(vinylamine), polyethylenimine, poly(vinylguanadine), poly(methylaminoethyl acrylamide), and poly(methylaminopropyl methacrylamide).
There is no particular restriction on the shape of the SAP particles used in this invention. The SAP particles can be in the form of spheres obtained by inverse phase suspension polymerization, flakes obtained by drum drying, or irregularly shaped particles obtained by pulverizing solid polymer. From the standpoint of the speed of absorption, the SAP particles preferably are small, and typically the particle size is about 20 to about 2000 xcexcm, preferably about 50 about 850 xcexcm.
The SAP particles, comprising an acidic resin, basic resin, a blend of acidic and basic resin, or multicomponent SAP particles, are surface treated by applying a surface crosslinking agent to the surface of the SAP particles, followed by heating the particles. Surface treatment results in surface crosslinking of the SAP particles. It has been found that surface treating SAP particles with an oxazolinium ion, either directly or through an oxazolinium ion precursor, enhances the ability of the SAP particles to absorb and retain aqueous media under a load.
In general, surface crosslinking is achieved by contacting SAP particles with a solution of an HAA or a stable oxazolinium ion that wets predominantly only the outer surfaces of the SAP particles. Surface crosslinking of the SAP particles then is performed, preferably by heating at least the wetted surfaces of the SAP particles. Heating the HAA forms oxazolinium ions in situ, which in turn surface crosslink the SAP particles. For oxazolinium ions having sufficient stability, the oxazolinium ion can be applied to the outer surfaces of the SAP particles, followed by heating the SAP particles to surface crosslink the exposed surface of the SAP particles.
The surface crosslinking agent utilized in the present invention is an oxazolinium ion. Simple oxazolinium ions have been isolated and characterized as disclosed in Stanssens et al., Proc.-Int. Conf. Org. Coat. Sci. Technol., p. 435 (1992). Reactions of oxazolinium ions with carboxylate anions to form esters of 2-hydroxyalkylamides have been disclosed in Winstein et al., J. Am. Chem. Soc., 72, p. 4669 (1950), and McLasland et al., J. Am. Chem. Soc., 72, p. 2190 (1950). Reactions of oxazolinium ions with carboxylic acids to form esters of 2-hydroxyalkylamides have been disclosed in Wicks et al., J. Coat. Tech., 57, p. 51 (1985).
The term xe2x80x9cstable oxazolinium ionxe2x80x9d is defined herein as an oxazolinium ion that is stable at room temperature and has sufficient reactivity with acidic (e.g., carboxyl) and basic (e.g., amino) moieties at about 90xc2x0 C. to about 170xc2x0 C. to form covalent bonds. Stability is imparted to an oxazolinium ion by judicious selection of a counterion, for example, a counterion comprising a conjugate base of an acid having a pKa of about 1.5 to about 6, preferably a pKa of about 2 to about 5, and most preferably about 2.5 to about 4.5. To achieve the full advantage of the present invention, the counterion is a conjugate base of an acid having a pKa of about 2.5 to about 3.5. Specific examples of counterions include, but are not limited to, benzoate, haloacetate, formate, fluoride, sulfonate (e.g., tosylate or mesylate), and the like.
The reactivity of a stable oxazolinium ion can be increased by exposing the stable oxazolinium ion to a sufficiently polar solvent (e.g., propylene glycol), optionally containing a sufficient concentration of a less acidic counterion to solvate or exchange the ion pair. The reactivity of a stable oxazolinium ion can be increased prior to heating in the presence of the SAP particles to facilitate the surface crosslinking of the exposed surface of the SAP particles.
The oxazolinium ion surface crosslinking agent utilized in the present invention can be generated in situ from a hydroxyalkylamine. Hydroxyalkylamides are disclosed in Swift et al. U.S. Pat. No. 4,076,917. An HAA useful in the present invention has the following formula: 
wherein A is a bond, hydrogen, or a monovalent polyvalent organic radical selected from the group consisting of a saturated or unsaturated alkyl radical contain 1 to 60 carbon atoms, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, eicosyl, triacontyl, tetracontyl, pentacontyl, hexylcontyl, and the like, aryl, for example, mono- and dicyclic aryl, such as phenyl, naphthyl, and the like, tri-C1-4 alkyleneamine, such as trimethyleneamino, triethyleneamino, and the like, and an unsaturated radical containing one or more ethylenic groups [ greater than Cxe2x95x90C less than ], such as ethenyl, 1-methylethenyl, 3-butenyl-1,3-diyl, 2-propenyl-1,2-diyl, carboxy C1-4 alkenyl, such as 3-carboxy-2-propenyl, and the like, C1-4 alkoxy carbonyl lower alkenyl, such as 3-methoxycarbonyl-2-propenyl, and the like; R1, selected independently, are hydrogen, straight or branched chain C1-5 alkyl, such as methyl, ethyl, n-propyl, n-butyl, sec-butyl, tert-butyl, pentyl, and the like, or straight or branched chain C1-5 hydroxyalkyl, such as hydroxyethyl, 3-hydroxypropyl, 2-hydroxypropyl, 4-hydroxybutyl, 3-hydroxybutyl, 2-hydroxy-2-methylpropyl, 5-hydroxypentyl, 4-hydroxypentyl, 3-hydroxypentyl, 2-hydroxypentyl, and the isomers of pentyl; R2, selected independently, are radicals selected from the group consisting of hydrogen and straight or branched C1-5 alkyl, or the R2 radicals can be joined to form, together with the carbon atoms, a cycloalkyl ring, such as cyclopentyl, cyclohexyl, and the like; p and pxe2x80x2, independently, are an integer 1 to 4; n is an integer having a value of 1 or 2, and nxe2x80x2 is an integer having a value of 0 to 2, or when nxe2x80x2 is 0, a polymer or copolymer (i.e., n has a value greater than 1, preferably 2-10) formed from the xcex2-hydroxyalkylamide when A is an unsaturated radical.
Preferred HAAs are wherein R1 is H or C1-5hydroxyalkyl, n and nxe2x80x2 are each 1, xe2x80x94Axe2x80x94is xe2x80x94(CH2)mxe2x80x94, m is 0-8, preferably 2-8, each R2 on the xcex1-carbon is H, and one of the R2 radicals on the beta carbon in each case is H and the other is H or a C1-5 alkyl, and q and qxe2x80x2, independently, are an integer 1 to 3; that is, 
Most preferred HAAs have the formula: 
wherein both R2 groups are H or both R2 groups are xe2x80x94CH3.
Specific examples of HAA compounds include, but are not limited to, bis[N,N-di(xcex2-hydroxyethyl)]adipamide, bis[N,N-di(xcex2-hydroxypropyl)]succinamide, bis[N,N-di(xcex2-hydroxyethyl)]-azelamide, bis[N-N-di(xcex2-hydroxypropyl)]adipamide, and bis[N-methyl-N-(xcex2-hydroxyethyl)]oxamide. A commercially available xcex2-HAA is PRIMID(trademark) XL-552 from EMS-CHEMIE, Dornat, Switzerland. PRIMID(trademark) XL-522 has the structure 
Another commercially available HAA is PRIMID(trademark) QM-1260 from EMS-CHEMIE, having the structure: 
An oxazolinium ion can be formed in situ by heating an HAA at a sufficient temperature and for a sufficient time. For example, an oxazolinium ion is formed from PRIMID(trademark) XL-552 as follows: 
The cyclic oxazolinium ion then reacts with a carboxyl or amino group on a polymer chain of an SAP to form a covalent bond. A PRIMID(trademark) XL-552 molecule then can form a second oxazolinium ion from a second xcex2-hydroxyethyl group on the molecule. This second oxazolinium ion reacts with carboxyl or amino group on a second polymer chain of the SAP, and, together with the first oxazolinium ion, forms surface crosslinks. Other HAA compounds provide corresponding oxazolinium ions.
The first and second oxazolinium ions generated in situ from an HAA molecule can be formed simultaneously. In addition, a stable oxazolinium ion can be applied directly to the surface of a SAP followed by heating to surface-crosslink the SAP.
In general, an oxazolinium ion utilized in the present invention, either stable or generated in situ has the following general structure: 
wherein B is 
and A, R1, R2, p, p1 and n are as defined above with respect to an HAA.
The SAP crosslinking solution typically contains about 0.01% to about 4%, by weight, of an oxazolinium ion precursor (e.g., an HAA) or a stable oxazolinium ion, and preferably about 0.4% to about 2%, by weight, oxazolinium ion precursor or oxazolinium ion, in a suitable solvent, for example, water, an alcohol, or a glycol. Examples of cosolvents that can be used include, but are not limited to, alcohols, e.g., methanol, ethanol, and isopropanol, and ketones, e.g., acetone. The solution can be applied as a fine spray onto the surface of freely tumbling SAP particles at a ratio of about 1:0.01 to about 1:0.5 parts by weight SAP particles to solution of oxazolinium ion or oxazolinium ion precursor.
To achieve the desired absorption properties, the HAA or the oxazolinium ion is distributed evenly on the surfaces of the SAP particles. For this purpose, mixing is performed in suitable mixers, e.g., fluidized bed mixers, paddle mixers, a rotating disc mixer, a ribbon mixer, a screw mixer, milling rolls, or twin-worm mixers.
The amount of HAA or oxazolinium ion used to surface treat the SAP particles varies depending upon the identity of SAP particles. Generally, the amount of HAA or oxazolinium ion, used to surface treat the SAP particles is about 0.001 to about 10 parts by weight per 100 parts by weight of the SAP particles. When the amount of HAA or oxazolinium ion exceeds 10 parts by weight, the SAP particles are too highly surface crosslinked, and the resulting SAP particles have a reduced absorption capacity. On the other hand, when SAP particles are surface crosslinked with less than 0.001 part by weight oxazolinium ion, there is no observable effect.
A preferred amount of HAA or oxazolinium ion used to surface crosslink the SAP particles is about 0.01 to about 5 parts by weight per 100 parts, by weight, SAP particles. To achieve the full advantage of the present invention, the amount of oxazolinium ion used as a surface crosslinking agent is about 0.05 to about 1 part, by weight, per 100 weight parts of SAP particles.
The formation of the oxazolinium ion surface crosslinking, and drying of the surface-treated SAP particles are achieved by heating the surface-treated particles at a suitable temperature, e.g., about 90xc2x0 C. to about 170xc2x0 C., and preferably about 100xc2x0 C. to about 165xc2x0 C. To achieve the full advantage of the present invention, the surface-treated particles are heated at about 100xc2x0 C. to about 160xc2x0 C. At this temperature, the SAP particles are surface crosslinked with the oxazolinium ion without degrading the color of the SAP particles and without increasing the residual monomer content of the SAP particles.
The surface-treated SAP particles are heated for about 60 to about 180 minutes, preferably about 60 to about 150 minutes, to effect surface crosslinking. To achieve the full advantage of the present invention, the SAP particles are heated for about 75 to about 180 minutes.
Ordinary dryers or heating ovens can be used for heating the surface-treated SAP particles and the oxazolinium ion precursor or oxazolinium ion. Such heating apparatus includes, for example, an agitated trough dryer, a rotating dryer, a rotating disc dryer, a kneading dryer, a fluidized bed dryer, a pneumatic conveying dryer, and an infrared dryer. However, any other method of forming or reacting the oxazolinium ion with the polymer of the SAP particle to achieve surface crosslinking of the SAP particles, such as microwave energy, can be used. In the surface treating and surface crosslinking steps, the mixer can be used to perform simultaneous mixing and heating of the HAA or oxazolinium ion, and the SAP particles, if the mixer is of a type that can be heated.
As previously stated, surface treating with an HAA or an oxazolinium ion, and subsequent or simultaneous heating, provides additional polymer crosslinks in the vicinity of the surface of the SAP particles. The gradation in crosslinking from the surface of the SAP particles to interior, i.e., the anisotropy of crosslink density, can vary, both in depth and profile. Thus, for example, the depth of surface crosslinking can be shallow, with a relatively sharp transition from a high level to a low level of crosslinking. Alternatively, for example, the depth of surface crosslinking can be a significant fraction of the dimensions of the SAP particle, with a broader transition.
Depending on size, shape, porosity, as well as functional considerations, the degree and gradient of surface crosslinking can vary within a given type of SAP particle. Depending on variations in surface:volume ratio within the SAP particles (e.g., between small and large particles), it is typical for the overall level of crosslinking to vary over the group of SAP particles (e.g., is greater for smaller particles).
Surface crosslinking generally is performed after the final boundaries of the SAP particles are essentially established (e.g., by grinding, extruding, or foaming). However, it is also possible to effect surface crosslinking concurrently with the creation of final boundaries. Furthermore, some additional changes in SAP particle boundaries can occur even after surface crosslinks are introduced.
The surface crosslinked SAP particles of the present invention can be used as an absorbent in disposable diapers, sanitary napkins, and similar articles, and can be used in other applications, for example, a dew formation inhibitor for building materials, a water-holding agent for agriculture and horticulture, and a drying agent.
Surface-crosslinked SAP particles of the present invention have advantages over conventional absorbent particles. The surface-crosslinked SAP particles of the invention can be produced at a low cost by a simple method which involves mixing SAP particles with an HAA (or other oxazolinium ion precursor) or a stable oxazolinium ion, and heating. The resulting surface-crosslinked SAP particles are less susceptible to fish eye formation than conventional absorbent resins and, therefore, exhibit a high rate of liquid absorption The present surface-crosslinked SAP particles also are white to off-white in color.